Antimony thioantimonate, Sb(SbS.sub.4), is useful as a lubricant additive. The compound is actually a "complex" compound in the sense that each of the two antimony atoms probably exist in different valence states, i.e. the +3 and +5 states.
Antimony thioantimonate (SbSbS.sub.4) has been prepared by reaction of antimony oxide (dissolved in cone potassium hydroxide solution) with sodium thioantimonate (Na.sub.3 SbS.sub.4) in an aqueous medium followed by neutralization of the resulting solution with an acid. See, for example, J. P. King and Yayesh Asmerom, "Investigation of Extreme-Pressure and Antiwear Properties of Antimony Thioantimonate", ASLE Transaction, Vol. 24, 4, 497-504 (1981); and U.S. Pat. No. 3,965,016, to Soulen (issued June 22, 1976). The overall reaction can be written as follows: ##STR1## There are severe limitations associated with the above reaction, i.e., (1) co-precipitation of free sulfur (6%) with the final product and (2) generation of hydrogen sulfide during neutralization. Subsequent studies have shown that a side reaction simultaneously occurs during the neutralization step, as follows: EQU 2Na.sub.3 SbS.sub.4 +6H.sup.+ .fwdarw.SbSbS.sub.4 +3H.sub.2 S+S6Na.sup.+( 2)
Excessive amounts (more than about 1%) of free sulfur in lubricant additives (such as SbSbS.sub.4 and others) are highly undesirable because the free sulfur promotes corrosion of copper-containing metal parts. Because of the added costs in removing free sulfur from the final product and collecting H.sub.2 S, an improved and economic manufacturing process for preparing SbSbS.sub.4 is highly desirable.
This invention involves a modified reaction route with straight-forward processing conditions to prepare SbSbS.sub.4 having a low sulfur content. The new process can produce SbSbS.sub.4 containing free sulfur at the more acceptable level of about 1% or less. Furthermore, generation of H.sub.2 S and other decomposition or products of hydrolysis are virtually eliminated. Moreover, the process is easily controlled and can be operated as a continuous process. The process is rendered even more highly economical as the halide stabilized antimony trihalide reactant can be prepared by treatment of Sb.sub.2 O.sub.3 with concentrated HCl and NaCl and then used, preferably in the form of NaSbCl.sub.4, as a reactant in the aqueous form of solution, without the severe hydrolysis and decomposition problems customarily associated with conventional antimony trihalides.
This invention includes a novel method for producing antimony thioantimonate, SbSbS.sub.4, by continuously feeding two reactants into a reactor and withdrawing the SbSbS.sub.4 therefrom. The reaction product is subsequently centrifuged, washed and dried as part of the continuous process. The use of a water-soluble, halide-stablized antimony trihalide complex as one of the starting materials enhances the economics of the process significantly and greatly facilitiates the overall processing operation, including enhanced yields and a low level of undesirable products of decomposition and/or side reactions.